1. Field of the Invention
This invention relates to an oxygenate additive for motor fuels and particularly to an additive containing a major proportion of isopropanol (IPA), some diisopropyl ether (IPE), and some water.
Recently enacted governmental regulations require that motor fuels such as gasoline be formulated in a manner to avoid production of certain noxious waste by-products discharged to the atmosphere when the fuel is burned in an engine. These regulations are directed toward the reduction of olefins and aromatics, and in order to implement such goal, require the presence of oxygen. In the United States, motor fuels for certain designated geographical areas must contain 2 weight percent (wt %) oxygen by Jan. 1, 1995 and 2.7 wt % oxygen by Nov. 1, 1992 in particular areas. Addition of oxygenates to gasoline is intended to reduce hydrocarbon and carbon monoxide exhaust emissions to a level which meets presently mandated emission standards.
2. Description of the Prior Art
a. Motor Fuel Oxygenate Additives
Alcohols have long been added to motor fuels such as gasoline to increase the octane rating of the fuel. Ethanol and methanol have been the principal alcohols employed for this purpose. Methanol has now been excluded as an additive for gasoline because of the unacceptable emission products which result when burned in the presence of a hydrocarbon fuel.
Ethanol on the other hand has properties which limit its use as a motor fuel additive, particularly if employed at levels which are effective in reducing unburned hydrocarbon and carbon monoxide combustion by-products. Ethanol, which has been dehydrated to an extent that the cost of the product is low enough to permit economic use of the alcohol as a gasoline additive, still contains an amount of water that causes the alcohol to be immiscible in the fuel. As a result, the ethanol additive tends to separate from the hydrocarbon fuel under certain ambient temperature conditions. Furthermore, gasoline while stored can accumulate additional quantities of water from the atmosphere. This exposure of the gasoline to additional water, can trigger phase separation of the alcohol from the fuel.
A co-solvent is required to prevent phase separation of the C.sub.1 and C.sub.2 alcohols from the fuel. Tertiary butyl alcohol (TBA) is an example of a co-solvent that has been used with C.sub.1 and C.sub.2 additives for motor fuels.
Present oxygenates used as motor fuel additives include methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME). These products proved to be expensive and required importation of methanol for production of MTBE and TAME, or ethanol for preparation of ETBE. Methanol or ethanol are not products which are conventionally produced in refinery processes, thus increasing the cost of the additive.
Isopropanol (IPA) has been proposed as an oxygenate additive for fuels, but available methods for preparation of IPA have limited its usefulness from a cost standpoint. Limited amounts of diisopropyl ether (IPE) removed from IPA during purification of the alcohol have found their way into motor fuels in Europe. IPE and IPA additions to the motor fuel though, were for deicing and dewatering purposes. In view of the fact that the IPA and/or IPE additives were incorporated for their dewatering properties, they did not include a proportion of water.
b. Early IPA Manufacturing Processes
IPA was first produced on a large scale in the United States commencing about 1920. The process involved hydration of propene with mineral acid. In the first stage, sulfuric acid half esters were produced. During the second stage, hydrolysis took place on a continuous basis. Co-product aqueous sulfuric acid was reconcentrated by submerged combustion. High boiling organic compounds present in the diluted acid were oxidized by addition of nitric acid. Small amounts of sulfuric acid were also purged.
This two-step process had several disadvantages. The more important ones were: the conditions of the reaction were corrosive by virtue of the fact that 85% sulfuric acid was necessary for the propene esterification reaction; it was necessary to dilute the acid reaction medium to promote hydrolysis and to facilitate separation of the acid from the product alcohol after the hydrolysis reaction; acid reconcentration was required before recycle to the esterification step of the process; and there was a frequent need to neutralize the product alcohol. The cost of acid reconcentration was a major factor in the economics of the process.
c. Direct Hydration of Olefins Over a Solid Catalyst
At least as early as the 1960's, direct hydration of olefins in the vapor phase over stable catalysts was proposed as a replacement for the long practiced, two-step, sulfuric acid hydration process. Catalysts that were suggested included silicophosphoric acid, phosphoric acid on Celite, and tungstic acid on aluminum. The Veba Chemie process using a fixed bed catalyst of supported phosphoric acid gained considerable commercial usage. These vapor phase, direct hydration processes overcame certain of the major difficulties of the previously practiced two-step acid esterification-hydrolysis process. However, these processes suffered from the disadvantage of low per-pass conversion to alcohol as a result of thermodynamic, vapor phase equilibrium conditions. The low perpass conversion necessitated relatively high recycle rates to obtain the same over-all conversion obtained from the older two-step sulfuric acid dehydration process.
During the 60's, studies were also conducted to determine the usefulness of sulfonated polystyrene-divinylbenzene resins as catalysts for the selective hydration of branched olefins to form alcohols. Among the strongly acidic, ion-exchange resins of the sulfonic acid type tested, were Amberlyst 15 and IR-120, at that time available from Rohm and Haas Co. It was determined that these resins did have the ability to catalyze the hydration of olefins to alcohols, but they first had to be treated with a strong acid before use and then washed with deionized water until residual acid could no longer be found.
d. Deutsche-Texaco A.G. Process
In the early 1970's, Deutsche-Texaco A.G. (DT) reported that it had built an IPA plant using ion exchange resins based on styrene, cross-linked with varying amounts of divinylbenzene. In the DT process, a mixture of liquid water and gaseous propene was fed to the top of a reactor having four separate catalyst beds. The water and gaseous reactant phases passed through the reactors in the same direction.
Quench water was introduced into each of the reactor beds for temperature control. Reaction temperatures of from about 270.degree. F. to about 302.degree. F. and pressures of from about 900 psia to about 1500 psia were maintained in the reactor beds. As the catalyst aged, the temperature of each bed was allowed to increase. Optimal molar ratios of water to propene were found to be from 12.5-15.0:1. It has been reported that polymers of propene formed when the amount of water used in the reaction mixture was below the optimal values. A conversion of up to 75% of the propene feed per-pass was said to have been realized.
The crude alcohol bottoms from the reactor unit were neutralized by addition of sodium hydroxide. IPE was stripped from the IPA-water mixture in a first distillation section. The pre-purification column bottoms from the first still were fed to another column where an azeotropic alcohol mixture was taken overhead. Final drying of the azeotrope alcohol was carried out utilizing benzene.
e. Tokyuama Soda IPA Process
In the latter part of the 1970's, Tokyuama Soda (TS) announced an IPA process carried out as a liquid-phase hydration of an olefin in the presence of a dissolved tungsten catalyst. Pulverized zirconium tungstate in an aqueous solution (polytungsten anion) was reported to be the most stable catalytic agent.
In the TS process, propene, water and recycled catalyst were preheated and pressurized before being introduced into the bottom of a reaction chamber. Reactor effluent from the upper end thereof was subjected to pressure reduction and separation of dissolved propene along with heat recovery. Unconverted propene separated by flash liberation was condensed for recycle to the reactor after purification.
An aqueous solution removed from the separator was sent to a purification section where the catalyst was removed and an azeotropic distillate containing 88% wt IPA with the remainder as water. The bottom-out water containing catalyst was recycled to the reaction section. A light-end column was provided to strip off low boiling constituents such as IPE and acetone, and the IPA was dried by benzene azeotropic distillation.